Broadband magnetron



April 2, 1957 A. T. NoRDslEcK BROADBAND MAGNETRON Filed June 10, 1949ARNOLD ATTORNEY BROADBAND MAGNETRoN Arnold Theodore Nordsieck,Champaign, Ill., assigner to International Standard ElectricCorporation, New York, N. Y., a corporation of Delaware Application June10, 1949, Serial No. 98,194

1 Claim. (Cl. S15-39.3)

This invention relates to broadband amplifiers for microwaves, and moreparticularly to a novel broadband amplifier of the magnetron type.

It is an object of the invention to provide a multi-cavity magnetrontype structure which is non-oscillating and which is capable ofamplifying over a wide frequency range.

The multi-cavity magnetron principle for generating micro-wave power isreadily convertible to broadband amplification in a way very similar tothat in which the velocity modulation principle has been applied in thetravelling wave amplifier. The most significant feature of thetravelling wave amplier is that the coupling between the electron streamand the R. F. fields is maintained continuously with `a definite phaserelation over a large number of complete R. F. cycles. From one point ofview however the multi-cavity magnetron may be regarded as an amplifierwhich is likewise several R. F. cycles long around the anodecircumference (n cycles long if n is the mode number). Under theseconditions the magnetron functions as a self-excited oscillator becauseit is made re-entrant upon itself. Any noise signal present in any partof the tube is amplified indefinitely by circulating around the tube,and the tube oscillates spontaneously. A small fraction of the powercirculating'in the device is bled ofi at one point in the circle, andthis constitutes the output power. The phase relation between theelectron stream and the R. F. field is maintained constant by adjustingthe applied D. C. voltage and magnetic field so that the mean rate ofcirculation of the electrons around the tube is equal to the angularphase velocity of the R. F. field (the so-called Hartree condition). Fora complete discussion of these ideas, reference may be made to the April1946 issue of the Bell System Technical Journal, and to the articletherein by Fisk, Hagestrum and Hartman, entitled The Magnetron as aGenerator of Centimeter Waves.

In the mode of highest n value, the so called ir-mode, the excitation inan oscillating magnetron is not a circulating wave propagating aroundthe anode structure but rather a standing wave pattern. ln this specialcase, widely used in oscillators, the qr-mode frequency is exactly thecut-off frequency of the transmission line forming the anode structure.ln modifying the magnetron to make it suitable for use as an amplifier,it is not desirable to operate near a cut-oft frequency of thetransmission line forming the anode, since this condition would involvegreat frequency sensitivity. It may be noted however that even in a1r-mode oscillation, where the excitation is necessarily a standing wavepattern, the electron stream is coupled appreciably only to thatcomponent of the pattern which represents a travelling wave circulatingwith the electron stream. The other component of the pattern, whichrepresents fan equal wave travelling against the electron stream, has anegligible effect on the electronic motions, infiuencing their orbitssomewhat but exchanging no net energy with them. This is because therelative phase of the electron stream and this latter field comnitedStates iPatent O 2387,?34 Patented Apr. a, 1957 ponent changes veryrapidly. These results are known from theoretical investigations byHartree and others. In magnetron oscillators operating in a non1r-rnode, it has been established theoretically and confirmedexperimentally by probe measurements in the resonator cavities that theexcitation on the anode structure is a true rotating wave. In convertingthe multi-cavity magnetron oscillator for use as an amplifier, it isaccordingly convenient to operate in a condition similar to the nonwmode.

In accordance with a feature of my invention I provide a magnetron typeamplifier structure having R. F. circuits electrically smooth andreflectionless over a wide band, in which the re-entrant characterassociated with the magnetron oscillator is not present and in whichthat part of the circuit which presents the radio frequency fields toelectron stream provides a phase velocity which is small compared to thefree space 'velocity of electromagnetic waves, and is independent offrequency over a Wide band.

The above-mentioned yand other object and features of the invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawings, wherein:

Fig. l is a schematic diagram illustrative of certain features of theinvention;

Fig. 2 is a sectional side view of an embodiment of I the invention; andFig. 3 is a sectional end View of the embodiment shown in Fig. 2.

Referring first to Fig. 1, I show in schematic form a section of alinear magnetron having an anode 1 and a cathode 2. separated by adistance d centimeters. A D.C. voltage difference of V volts is appliedbetween anode and cathode, and a magnetic field of B gauss is directedperpendicular to the plane of the section. A linear magnetron is shownfor purposes of illustration because, since the magnetron is used as anamplifier, it cannot be re-entrant-and a linear magnetron is thesimplest alternative to :a re-entrant circulating magnetron. Moreover,the formulae which express the behavior of the magnetron are simplestfor the linear case.

We consider the behavior of the magnetron of Fig. l when amplifying awave having a free space wave length A0. The symbol )tu (shown in Fig.l) is used to denote transmission line wave length, or the distancealong the anode corresponding to one complete cycle of the R. F. fields.The number` ha 7k is then determined by the anode geometry, and is muchless than unity. The formulas which are used in the followingdescription are taken from the paper previously gauss The unit currentI0 is given by 8 I 17 amperes per space charge spoke where W is thewidth of the anode.

The maximum electronic efficiency is given by In order to obtain anoverall efficiency of approximately 50%, au electronic eciency of. 6.0%is.` sufficient, since losses in the structure are small. Thiscorresponds to a value for V/ V of about 8. Assuming that Vo=160 volts,the required voltage V is.1280'volts,

and

For a midband free space wavelength o=6 cm., the guide wavelength Aabecomes 6X.025=0.15 cm.

The anode-cathode spacing d may be found from the empirical rule that`i=07 approximately which has been found to give'good results in othermagnetrons. Thus d=0.67(0.15),=0.10 cm.=.04 inch The unitmagnetic.tield` B0 is To find B, it is noted. thatY B/B0=1/z (V/.V0-I-1)=4.5

hence B: (4.5)(425.)=1920 gauss The current per space charge spoke is pamperes= 15 ma.

where W has been assumed to be. 0.15 inch. Assuming that unit currenttioWs, the maximum generated R. F. power per spoke at the outputV end isFor an output power level of 200 watts, the gain per spoke is Since morethan one unit of current canbe drawn, a gain of 0.5 db per spoke isreasonable. For a total gain of 20 db, therefore, 40 spokes arerequired. This requires that the anode have a length of The anodemayconveniently be` formed offa helix` having a pitch of 1A to 1/6 ha; asan example 11)\I=0.04cm. The circumference of the helix is'. theMiko-:1.5 cm., so that the helix diameter is In Figs. 2 and 3, I show anillustrative embodiment of my invention. The R. F. structure is enclosedin a suitable evacuated vessel. 3. The input signal is applied i issupplied to cathode 6 via lines 7. The' D. C. electric eld is appliedbetween-one of lines 7 and anode 5, as shown. The amplified outputenergy is coupled via impedance matching devices` (not shown) tocoaxialline 8.

In Fig. 3 a section of the amplifier, along lines 3-3 i high, is shownin which correspondingl parts are given like reference numerals. Inaddition magnetic poles 9 and 10 which supply the transverse magneticfield are shown.

While I have described `above the principles of my in vention inconnectionV with' a particularembodiment, it is to be clearly understoodthat this description is made only by way of example and not as alimitation tothe scope of my invention; In particular the invention isnot limited to a linear magnetron, as it is apparent that otherstructures might be used. For example, a circular magnetron which is not`re-entrant and in which the input and output circuits are isolated fromone another may be used. Again, the invention is not limited to thehelical type of anode, as other types of known transmission lines onwhich waves propagate at reduced velocity may be used.

What I claim is:

Inl a broadband' microwave amplier, a two element substantiallylrectilinear transmission line forl propagating electromagnetic waves,comprising two spaced conductive elements, one of said elementscomprising an inductively loaded anode consisting essentially ofk ahelically shaped metallic structure, and the other of said elementscomprising an electron emissive cathode extending along the length ofsaid line, means for producing a unidirectional electric eld betweensaid two elements, means for producing between said elements a magneticfield perpendicular to said electricfeld, and to said linemeans forapplying radio frequency energy to one end of said transmission line andoutput means coupled to the other end of said transmission line.

References Cited in the tile of this patent UNITED' STATES PATENTS2,112,538 Potter July 5, 1938 2,241,976 Blewettet al May 13, 19412,300,052 Lindenblad Oct. 27, 1942 2,367,295 Llewellyn Ian. 16,` 19452,414,121. Pierce Ian. 14, 1947 2,428,612 Blewett Oct. 7, 1947 2,439,401Smith Apr. 13, 1948 2,511,407 Kleen et al June 13, 1950 2,566,087 LerbsAug. 28, 1951 2,582,185 Willshaw Jan. 8, 1952

